Non-returnable packages for beverages and other contents are manufactured at present in general from a laminated material comprising a carrier layer of fibrous material, e.g. paper, which is surrounded on either side by thin, thermoplastic layers. The thermoplastic layers render the packing container liquid-tight and make possible, moreover, the heat-sealing of longitudinal joints, top and bottom. Many different types of packing containers of this type are known. Such packing containers are manufactured also in aseptic form for being filled with previously sterilized contents, e.g. heat-sterilized milk. The packing container here has to be provided with a gas-tight barrier layer, and in general the packing laminate to this end comprises a thin layer of aluminium foil or some plastic material of high gas-tightness. In this manner the packing material can be made completely gas-tight and bacteria-tight. It is not sufficient, however, to make the actual material bacteria-tight but, naturally, the joints and seals which are required for converting the material to a finished packing container must also be completely gas-tight. This is a major problem in certain types of known packing containers. This is true in particular for openable top parts on so-called gable-top packages, since these packages generally are opened by breaking the top seal itself, and there is thus the choice between providing a strong seal which is gas-tight but difficult to open, or a weaker seal which is easy to open but may entail a risk of leakage. The reason that these problems are particularly marked in packing containers of the gable-top type is that the top here comprises two bellows-folds folded against each other, which are located between two outer sealing panels and which are sealed together with these. Between the points of the bellows-folds directed against each other a channel is formed, owing to the thickness of the material which is difficult to make completely bacteria-tight. Since the packing container is opened by breaking the seal between different parts of the sealing fin and the bellows-fold, it is necessary, moreover, purposely to limit the strength of the seal if the opening function of the package is to be retained. Attempts to produce an aseptic packing container of the gable-top type have been unsuccessful up to now, largely because of these difficulties.